In recent years, in accordance with the advance in optical technology, various types of display technologies such as plasma display panels (PDP), liquid crystal displays (LCD), organic/inorganic EL displays (ELD) and the like have been suggested and sold in the market instead of a known cathode-ray tube. In the above-mentioned displays, the use of various types of plastic films has been suggested and the required characteristics thereof have been sophisticated. For example, in the case of the liquid crystal displays, in order to obtain the slimness and the lightness and to improve display characteristics, various types of plastic films are used in a polarizing plate, a retardation film, a plastic substrate, a light guide plate.
In the case of the polarizing plate, in general, the polarizing plate has a structure in which the triacetyl cellulose film (hereinafter, referred to as TAC film) acting as the protective film is laminated by using the aqueous adhesive made of the polyvinyl alcohol aqueous solution on the polarizer. However, both the polyvinyl alcohol film used as the polarizer and the TAC film used as the protective film for polarizers have poor heat resistance and humidity resistance. Therefore, if the polarizing plate that is made of the films is used at a high temperature or a high humidity atmosphere for a long period of time, the degree of polarization is reduced, the polarizer and the protective film are separated from each other, or optical properties are reduced. Thus, in terms of the purposes, there are various limits. In addition, in the case of the TAC film, a change in in-plane retardation (Rin) and thickness retardation (Rth) is significant according to a change in the ambient temperature/humidity atmosphere. In particular, a change in retardation in respects to incident light at an inclined direction is significant. If the polarizing plate that includes the TAC film having the above-mentioned characteristics as the protective film is applied to a liquid crystal display, the viewing angle characteristics are changed according to the change in the ambient temperature/humidity atmosphere, thus reducing the image quality. In addition, in the case of the TAC film, a change in dimension is significant according to the change in the ambient temperature/humidity atmosphere and the photoelastic coefficient is relatively high, thus, after evaluation in respects to durability in a heat resistant and humidity resistant atmosphere, a change in retardation characteristic occurs locally, thereby easily reducing the image quality.
As a material to complement the disadvantages of the TAC film, a methacryl resin is well known. However, it is known that the methacryl resin is easily broken or splited to cause problems in terms of transportability during the production of the polarizing plate and the productivity is poor. In addition, in the case of when the acrylate resin is used as the material of the film, since it is required that a casting process is used, there are problems in that it is difficult to perform the production process and the cost is high.
In the case of the retardation film, the film that is made of the styrene resin is a material that shows the optical anisotropic property that the refractive index is increased perpendicular to the alignment direction when the film is stretched and aligned, and it is known that the film is stretched to be used for manufacturing the film having the positive thickness retardation (Rth). In addition, there are advantages in that the styrene resin has the excellent economic efficiency and transparency. However, there are problems in that the heat resistance is insufficienty and the mechanical properties are poor except for the case of when the costly special monomer is used to perform the manufacturing. In addition, in the case of when the polycarbonate resin is stretched to manufacture the retardation film, it is possible to provide the sufficient retardation function, but the change in retardation is significant in respects to the degree of stretching and it is difficult to provide the film having the uniform and stable retardation function.